There are a number of valve applications for metering the flow of a liquid that require a high degree of controllable flow regulation and require a complete cut-off with low leakage. However, valve structures that can meet one requirement often can not easily meet the other, particularly in a manner that is economically feasible for many common valve applications.
Consider fuel supply metering in the context of a diesel engine. Some diesel engine manufacturers provide a flow-metering valve in the fuel line, for example, in the inlet line going to the high pressure fuel pump. The valve regulates fuel flow to the pump, and the pump in turn provides fuel under pressure to an accumulator, and thence to fuel injectors coupled to the cylinders of the engine, for combustion. The flow regulation allows the diesel engine to run more efficiently, because it is not using horsepower to build pressure in the high pressure flow for what would otherwise be unused diesel fuel.
To perform the flow regulation, the metering valve must provide a fine degree of flow control and corresponding accurate responsiveness to a control signal. Poppet type valves do not provide the necessary regulation performance. A poppet valve has a moveable valve member with a face that engages a seat, when the moveable valve member reaches the closed position. Withdrawal of the poppet valve face away from the seat opens the valve. By nature, with a poppet design, a small degree increase in opening produces a large gain in the valve-opening area, and a corresponding large increase in flow. Hence, prior designs for fuel regulation valves in the inlet lines of diesel engines have used spool and sleeve designs.
Spool valve designs use a sleeve with lateral flow openings and a spool mounted for sliding axial movement within the sleeve. In a closed position, the sidewalls of the spool completely block the flow openings. As the spool moves along the sleeve axis, the movement opens access to the flow openings, for example, by withdrawing the spool past the flow openings or by aligning passages or notches in the spool with the flow openings through the sleeve. The positioning movement of the spool can be readily controlled by the current applied to a solenoid actuator Precise positioning of the spool within the sleeve provides a fine degree of control of the amount that the flow ports are opened and thus control of the amount of fluid flowing through the valve.
However, at times, it also is appropriate to control the metering valve in the diesel engine fuel supply application to completely cut-off the supply of fuel, for example, to starve the engine of fuel and thus cut-off combustion in the diesel engine. It is difficult to achieve a high degree of cut-off (low or no leakage) using spool valve designs. Because of the need for the spool to move within the sleeve, there must be some clearance between the outer surface(s) of the spool and the inner surface(s) of the sleeve. If the clearance is at all substantial, fluid tends to leak past the spool in the fully closed condition. Past efforts to minimize leakage at cut-off have relied on constructing spool valves, for fuel inlet metering, so that the tolerances between the spool and the sleeve are extremely tight (though not too tight since improper dimensioning causes friction and wear or even binding). Often, the assembly process has involved manually fitting individual spools and sleeves together and testing the tolerances and/or leakage. Such an approach makes manufacture of the metering valves extremely complex and expensive.
Also, it is often difficult to build a large number of such valves that satisfy strict performance standards, e.g. degree of opening (flow) at zero-current and/or response to a specified current. Minor differences in the dimensions of the valve elements and/or in the solenoid-based actuator could result in substantial variation from valve to valve in the finished products. It was also not practical to adjust or adapt a given valve design for use in different specific applications, for example, to provide different performance for different sized diesel engines.
Hence a need exists for an improved liquid flow metering valve for applications, such as inlet metering of fuel to a diesel engine, which require a high degree of controllable flow regulation as well as cut-off with low leakage. It is desirable that any such valve structure be easy and inexpensive to manufacture and yet provide minimal performance variation from valve-to-valve. If practical, the design should be readily adaptable to use in multiple applications requiring-different performance characteristics.